涉及源碼（Android 4.4.2）：
frameworks/base/cmds/app_process/app_main.cpp
/frameworks/base/core/jni/AndroidRuntime.cpp
/libnativehelper/JniInvocation.cpp
/dalvik/vm/Jni.cpp
/dalvik/vm/JniInternal.h
/libnativehelper/include/nativehelper/jni.h

首先來(lái)看看app_main.cpp的main方法，它是啟動(dòng)虛擬機(jī)的入口方法。

frameworks/base/cmds/app_process/app_main.cpp

int main(int argc, char* const argv[])
{
   // zygote進(jìn)程是由init進(jìn)程通過fork而來(lái)
    AppRuntime runtime;
  
    if (zygote) {
        runtime.start("com.android.internal.os.ZygoteInit",
                startSystemServer ? "start-system-server" : "");
    }
}

/frameworks/base/core/jni/AndroidRuntime.cpp

void AndroidRuntime::start(const char* className, const char* options)
{

    /* start the virtual machine */
    JniInvocation jni_invocation;
    // 1、初始化JNI相關(guān)的函數(shù)
    jni_invocation.Init(NULL);
    JNIEnv* env;
    // 2、啟動(dòng)虛擬機(jī)
    if (startVm(&mJavaVM, &env) != 0) {
        return;
    }
    onVmCreated(env);

    // 3、注冊(cè)JNI函數(shù)
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
}

一、初始化JNI相關(guān)的函數(shù)

/libnativehelper/JniInvocation.cpp

bool JniInvocation::Init(const char* library) {
#ifdef HAVE_ANDROID_OS
  char default_library[PROPERTY_VALUE_MAX];
  property_get(kLibrarySystemProperty, default_library, kLibraryFallback);
#else
  const char* default_library = kLibraryFallback;
#endif
  if (library == NULL) {
    library = default_library;
  }
  // 通過dlopen加載libdvm.so，libdvm.so是dalvik vm的核心庫(kù)
  handle_ = dlopen(library, RTLD_NOW);
  //從libdvm.so中找到JNI_GetDefaultJavaVMInitArgs、NI_CreateJavaVM和JNI_GetCreateJavaVMs這三個(gè)函數(shù)指針。
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetDefaultJavaVMInitArgs_),
                  "JNI_GetDefaultJavaVMInitArgs")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_CreateJavaVM_),
                  "JNI_CreateJavaVM")) {
    return false;
  }
  if (!FindSymbol(reinterpret_cast<void**>(&JNI_GetCreatedJavaVMs_),
                  "JNI_GetCreatedJavaVMs")) {
    return false;
  }
  return true;
}

二、啟動(dòng)虛擬機(jī)

int AndroidRuntime::startVm(JavaVM** pJavaVM, JNIEnv** pEnv)
{
 // 前面是初始化操作，重點(diǎn)是整個(gè)方法
 if (JNI_CreateJavaVM(pJavaVM, pEnv, &initArgs) < 0) {
        ALOGE("JNI_CreateJavaVM failed\n");
        goto bail;
    }
}

JNI_CreateJavaVM方法就是前面libdvm.so中的方法，它就是創(chuàng)建虛擬機(jī)的方法。

下面來(lái)看看這個(gè)JNI_CreateJavaVM方法

/dalvik/vm/Jni.cpp

jint JNI_CreateJavaVM(JavaVM** p_vm, JNIEnv** p_env, void* vm_args) {
    const JavaVMInitArgs* args = (JavaVMInitArgs*) vm_args;
    
    //1、gDvm是一個(gè)靜態(tài)的數(shù)據(jù)結(jié)構(gòu)，數(shù)據(jù)類型為結(jié)構(gòu)體DvmGlobals
    // 虛擬機(jī)的大部分?jǐn)?shù)據(jù)都是有它來(lái)存放
    memset(&gDvm, 0, sizeof(gDvm));

    //2、創(chuàng)建JavaVMExt對(duì)象，其實(shí)也是JavaVM對(duì)象
    JavaVMExt* pVM = (JavaVMExt*) calloc(1, sizeof(JavaVMExt));
    pVM->funcTable = &gInvokeInterface;
    pVM->envList = NULL;
    dvmInitMutex(&pVM->envListLock);

   
    gDvmJni.jniVm = (JavaVM*) pVM;

   // 3、創(chuàng)建JNIEnvExt對(duì)象，其實(shí)對(duì)應(yīng)的也是JNIEnv對(duì)象
    JNIEnvExt* pEnv = (JNIEnvExt*) dvmCreateJNIEnv(NULL);

    //4、啟動(dòng)虛擬機(jī)
    gDvm.initializing = true;
    std::string status =
            dvmStartup(argc, argv.get(), args->ignoreUnrecognized, (JNIEnv*)pEnv);
    gDvm.initializing = false;
    
    *p_env = (JNIEnv*) pEnv;
    *p_vm = (JavaVM*) pVM;
   
}

1、重置gDvm結(jié)構(gòu)

DvmGlobals，該結(jié)構(gòu)體是Dalvik的核心數(shù)據(jù)結(jié)構(gòu)，幾乎所有的重要成員等都通過gDvm來(lái)管理。

2、創(chuàng)建JavaVMExt對(duì)象

JavaVMExt對(duì)象的funcTable成員用來(lái)保存Dalvik虛擬機(jī)實(shí)例函數(shù)表，從代碼可以看到，這個(gè)函數(shù)表為gInvokeInterface，gInvokeInterface是一個(gè)類型為JNIInvokeInterface的結(jié)構(gòu)體。

/dalvik/vm/jni.cpp

static const struct JNIInvokeInterface gInvokeInterface = {  
    NULL,  
    NULL,  
    NULL,  

    DestroyJavaVM,  
    AttachCurrentThread,  
    DetachCurrentThread,  

    GetEnv,  

    AttachCurrentThreadAsDaemon,  
};  

3、創(chuàng)建JNIEnvExt對(duì)象

下面來(lái)看看dvmCreateJNIEnv函數(shù)

JNIEnv* dvmCreateJNIEnv(Thread* self) {
    JavaVMExt* vm = (JavaVMExt*) gDvmJni.jniVm;
    
    // 1、創(chuàng)建JNIEnvExt對(duì)象
    // Dalvik虛擬機(jī)里JNI的所有函數(shù)都封裝在gNativeInterface中
    JNIEnvExt* newEnv = (JNIEnvExt*) calloc(1, sizeof(JNIEnvExt));
    newEnv->funcTable = &gNativeInterface;
    if (self != NULL) {
        dvmSetJniEnvThreadId((JNIEnv*) newEnv, self);
        assert(newEnv->envThreadId != 0);
    } else {
        newEnv->envThreadId = 0x77777775;
        newEnv->self = (Thread*) 0x77777779;
    }
    if (gDvmJni.useCheckJni) {
        dvmUseCheckedJniEnv(newEnv);
    }

    ScopedPthreadMutexLock lock(&vm->envListLock);

    // 2、將JNIEnvExt插入到JavaVMExt對(duì)象的成員變量envList列表的頭部
    newEnv->next = vm->envList;
    assert(newEnv->prev == NULL);
    if (vm->envList == NULL) {
        // rare, but possible
        vm->envList = newEnv;
    } else {
        vm->envList->prev = newEnv;
    }
    vm->envList = newEnv;
    
    return (JNIEnv*) newEnv;
}

（1）結(jié)構(gòu)體JNINativeInterface用來(lái)描述一個(gè)本地接口表。當(dāng)我們需要在C/C++代碼在中調(diào)用Java函數(shù)，就要用到這個(gè)本地接口表

static const struct JNINativeInterface gNativeInterface = {  
    ......  
      
    FindClass,  
  
    ......  
  
    GetMethodID,  
      
    ......  
  
    CallObjectMethod,  
      
    ......  
  
    GetFieldID,  
  
    ......  
      
    SetIntField,  
      
    ......  
  
    RegisterNatives,  
    UnregisterNatives,  
  
    ......  
  
    GetJavaVM,  
  
    ......  
};

（2）每一個(gè)Dalvik虛擬機(jī)實(shí)例還有一個(gè)JNI環(huán)境列表,這個(gè)列表保存JavaVMExt對(duì)象的成員變量envList中。JavaVMExt的JNI環(huán)境列表就是JNIEnvExt列表。每一個(gè)Attach到Dalvik虛擬機(jī)中去的線程都有一個(gè)對(duì)應(yīng)的JNIEnvExt，用來(lái)描述它的JNI環(huán)境。

他們之間的關(guān)系圖如下圖所示：

/dalvik/vm/JniInternal.h
/libnativehelper/include/nativehelper/jni.h

vmstart.png

4、初始化并啟動(dòng)虛擬機(jī)

dalvik/vm/init.cpp

std::string dvmStartup(int argc, const char* const argv[],
        bool ignoreUnrecognized, JNIEnv* pEnv)
{
     /*
     * Initialize components.
     */
    dvmQuasiAtomicsStartup();
    if (!dvmAllocTrackerStartup()) {
        return "dvmAllocTrackerStartup failed";
    }
    if (!dvmGcStartup()) {
        return "dvmGcStartup failed";
    }
    if (!dvmThreadStartup()) {
        return "dvmThreadStartup failed";
    }
    if (!dvmInlineNativeStartup()) {
        return "dvmInlineNativeStartup";
    }
    if (!dvmRegisterMapStartup()) {
        return "dvmRegisterMapStartup failed";
    }
    if (!dvmInstanceofStartup()) {
        return "dvmInstanceofStartup failed";
    }
    if (!dvmClassStartup()) {
        return "dvmClassStartup failed";
    }

    if (!dvmFindRequiredClassesAndMembers()) {
        return "dvmFindRequiredClassesAndMembers failed";
    }

    if (!dvmStringInternStartup()) {
        return "dvmStringInternStartup failed";
    }
    if (!dvmNativeStartup()) {
        return "dvmNativeStartup failed";
    }
    if (!dvmInternalNativeStartup()) {
        return "dvmInternalNativeStartup failed";
    }
    if (!dvmJniStartup()) {
        return "dvmJniStartup failed";
    }
    if (!dvmProfilingStartup()) {
        return "dvmProfilingStartup failed";
    }

    if (!dvmCreateInlineSubsTable()) {
        return "dvmCreateInlineSubsTable failed";
    }

    if (!dvmValidateBoxClasses()) {
        return "dvmValidateBoxClasses failed";
    }

    if (!dvmPrepMainForJni(pEnv)) {
        return "dvmPrepMainForJni failed";
    }

    if (!dvmInitClass(gDvm.classJavaLangClass)) {
        return "couldn't initialized java.lang.Class";
    }

    if (!registerSystemNatives(pEnv)) {
        return "couldn't register system natives";
    }

    if (!dvmCreateStockExceptions()) {
        return "dvmCreateStockExceptions failed";
    }

    if (!dvmPrepMainThread()) {
        return "dvmPrepMainThread failed";
    }

    if (dvmReferenceTableEntries(&dvmThreadSelf()->internalLocalRefTable) != 0)
    {
        ALOGW("Warning: tracked references remain post-initialization");
        dvmDumpReferenceTable(&dvmThreadSelf()->internalLocalRefTable, "MAIN");
    }

    if (!dvmDebuggerStartup()) {
        return "dvmDebuggerStartup failed";
    }

    if (!dvmGcStartupClasses()) {
        return "dvmGcStartupClasses failed";
    }

    /*
     * Init for either zygote mode or non-zygote mode.  The key difference
     * is that we don't start any additional threads in Zygote mode.
     */
    if (gDvm.zygote) {
        if (!initZygote()) {
            return "initZygote failed";
        }
    } else {
        if (!dvmInitAfterZygote()) {
            return "dvmInitAfterZygote failed";
        }
    }

}

主要用來(lái)初始化Dalvik虛擬機(jī)

三、注冊(cè)JNI函數(shù)

/frameworks/base/core/jni/AndroidRuntime.cpp

/*static*/ int AndroidRuntime::startReg(JNIEnv* env)
{
    // 注冊(cè)JNI函數(shù)，gRegJNI是一個(gè)全局?jǐn)?shù)組
    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL);
        return -1;
    }
    env->PopLocalFrame(NULL);

    return 0;
}

static const RegJNIRec gRegJNI[] = {
    REG_JNI(register_android_debug_JNITest),
    REG_JNI(register_com_android_internal_os_RuntimeInit),
    REG_JNI(register_android_os_SystemClock),
    REG_JNI(register_android_util_EventLog),
    REG_JNI(register_android_util_Log),
    REG_JNI(register_android_util_FloatMath),
    REG_JNI(register_android_text_format_Time),
    REG_JNI(register_android_content_AssetManager),
    REG_JNI(register_android_content_StringBlock),
    REG_JNI(register_android_content_XmlBlock),
    REG_JNI(register_android_emoji_EmojiFactory),
    REG_JNI(register_android_text_AndroidCharacter),
    REG_JNI(register_android_text_AndroidBidi),
    REG_JNI(register_android_view_InputDevice),
    REG_JNI(register_android_view_KeyCharacterMap),
    REG_JNI(register_android_os_Process),
    REG_JNI(register_android_os_SystemProperties),
    REG_JNI(register_android_os_Binder),
    // ...還是很多
    }

參考文章：
http://www.infoq.com/cn/articles/android-in-depth-dalvik
http://blog.csdn.net/luoshengyang/article/details/8885792